Mass spectroscopes having means for focusing the ion beam in synchronism with the scanning voltage



Feb. 4, 1969 c BRUNNEE ET AL 3,426,192 7 MASS SPECTROSCOPES HAVING MEANS FOR FOCUSING THE ION BEAM IN SYNCHRONISM WITH THE SCANNING VOLTAGE Filed Sept. 5, 1965 [NVENTOR-S C URT BRUNNLL 9/ BY GUNTER KAPPUS United States Patent 2 Claims ABSTRACT OF THE DISCLOSURE A mass spectroscope has an exit slit through which ions leave the ion source and follow a circular path determined by the ion source acceleration voltage toward one of two separated entrance slits corresponding to respective ones of the circular paths. A split lens electrode for focusing the ion beam upon the exit slit has a predetermined potential difference across the two lens halves to deflect the ion beam so that it passes through the exit slit. This poential difference is varied in accordance with the acceleration potential so that regardless of the mass range being scanned, ions within that mass range are always emitted through the exit slit.

The invention relates to a mass spectroscope in which a magnetic field is effective for focusing the electron beam in the ionisation chamber, scanning the masses being effected by alteration of the ion acceleration voltage, two or more ion interceptors being provided at different path radii.

At the basis of the present invention lies the observation that the ions are already deflected in the ion source by the field of the ion source magnet, so that a mass discrimination already occurs at the exit gap of the ion source.

The mass dispersion by the ion source magnetic field can be compensated by differently large voltages on both lens halves, but this compensation is correct only for a predetermined deflection radius in the separating magnetic field, so that in systems with two or more interceptors, difficulties arise as a result of the differently large deflection radii and deflection paths. According to the invention even in this case the influence of the mass dispersion in the ion source magnetic field can be effectively compensated in that in the potential field of the ion path, a transverse component is switched over, together with switching over onto the other interceptor. The differently long ion paths influence the quality of the Z-focusing in the direction perpendicular to the deflection plane. In order to prevent this, the focusing in axial direction can be adapted to the changing lengths of the ion paths, by switching over.

The above and other objects and advantages of the invention will be clear from the following description taken with reference to the accompanying drawing, which are given by way of example and in which the sole figure diagrammatically illustrates a mass spectroscope according to the invention.

The mass spectroscope illustrated operates in a known (manner, with electric mass scanning. It comprises a vacuum container 1 with an ion source 2, into which the substance to be analyzed is introduced in gaseous or vapour form e.g. from a sample storage container 3.

The ion source consists of a metal box 4 through which an electron beam A for impact ionisation passes between electrodes 5a, 5b. For focusing the electron beam the field of a magnet 6 serves, which in a manner similar to the electron beam A with the electrodes 5a, 5b, is in reality displaced through 90 relative to the plane of the drawing, so that its field is in register with the stray field of the separation magnets in the region of the electron beam A. The ions formed in the ionisation chamber 7 are drawn from the box 4 by a draw-out voltage U which lies on the electrode 8, through a window 9 in this electrode, and are guided through further electrodes 10 serving for accelerating and focusing, having lens voltages U and 11, so that focusing of the ion beam B takes place on the entry gap 12 in the sector field of the magnet 13. The focusing takes place mainly in the X-direction. In order to achieve a certain [bundling or focusing in the Z-direction which is perpendicular to the plane of the drawing, the

electrodes 11b, are provided.

In the sector field of the magnet 13, the ions of the ion beam B are deflected. The amount of this deflection depends on the strength of the magnetic field, the ion mass and the ion acceleration voltage between the metal box 4 and the exit electrode 11. With constant magnetic field and constant acceleration voltage U the deflection is determined solely by the mass number of the ions.

For separately intercepting the ions according to their mass number, two exit diaphragms 14a and 14b each with an exit gap 15a and 15b, and two electrodes 16a and 16b serving as interceptors are provided, which are connected to an anrangement 17 for measurement of the intercepted ion current. By alteration of the acceleration potential U all ions of the specimen, with their different tmass numbers, are successively brought onto the interceptors 16a and 16b respectively and the measurement is thus effected qualitatively and quantitatively in the arrangement 17. The interceptor 15a for the mass range 2 to 12 is provided for the deflection radius 2.6 cm. and the interceptor for the mass range 12 to is provided for the deflection radius 6.5 cm.

For the ion source, the following voltage supply is provided:

Draw-out voltage U on the electrode 8.Constant voltage U adjustable between 0 and 3 volts, together with a co-varying voltage adjustable between 0 and 8.10

U (U =24 volts for U =3kv.).

Lens voltage U on the lens 10.Average value fixedly set to about U =8.1O U (U =24 volts for U =3 kv.). Dilference voltage U between the two lens halves adjustable between 0 and 16.10 U (U -=48 volts for U =3 kv.). Difference for both interceptors 16a, 16b separately adjustable. Change-over of the difference voltage U on transition from one interceptor to the other.

Deflection voltage for axial focusing (Z-focusing) on the electrode 11b.Absolute value adjustable for interceptor with r=2.6 cm. between 36.1O U (U ,=l080 volts for U =3 kv.) and 45.10 U (U =O volts for U =3 kv.), for interceptor with r=6.5 cm. between 25.10 U (750 volts/3 kv.) and 36.10 U =(1080 volts/ 3 kv.). For each interceptor the difference voltage U is adjustable between 0 and 6.lO U (U =l80 volts for 3 kv.). Change-over of the absolute and difference voltages on transition from one interceptor :to the other.

This voltage supply is so arranged that the ion space charge, electron space charge, the magnetic field provided for bundling the electron beam and the Z-focusing can be allowed for in a favourable manner for the measurement.

The following explanation will enable the relationships to be understood:

As a result of the electric mass scanning the relationships are as follows: For a given mass m and thus a predetermined acceleration voltage, the transverse deflection caused by the magnetic field in the ion source can be compensated, e.g. by suitable offset of the ion source gap, or electrically by a different voltage U on the two lens halves, so that the ion bundle of the mass m passes through the end gap 12 of the ion source, and that on alteration of the acceleration voltage also a different mass m is focused on the interceptor, even if the path of the ion bundle belonging to mass m is changed in the ion source magnetic field.

The mass dispersion in the ion source magnetic field can thus be fully compensated so that differently large masses m and M1 pass in a similar manner through the end gap of the ion source despite the mass dispersion in the ion source.

If now, as is the case in the device described, two ion interceptors 16a, |16b at different radii r r are used, then compensation of the mass dispersion causes difliculties since by a certain offset of the gap or by a predetermined voltage difference on the lens halves, the mass dispersion can be removed only for a single ion path, i.e. for a single interceptor. Practical measurements have confirmed this.

For compensating the mass dispersion it is therefore necessary on transition from one interceptor to the other, to also switch over the lens voltage difference.

Furthermore, it is important also to change over the deflection, which causes focusing in the Z-direction (direction of the magnetic field) after the end gap of the ion source. The ion paths to both interceptors have different lengths, e.g. corresponding to 11:26 cm. and r =6.5 cm. In order that the point of most favourable Z-focusing lies always in the range of the interceptor in question (greatest ion current), the voltage on the deflection electrodes, which determines the position of the Z-focus region, must be different for both ion paths. Practical measurements have confirmed this.

We claim:

1. A mass spectroscope comprising:

an ionisation chamber and a magnet for focusing the electron beam in the ionisation chamber,

means defining an exit slit through which an ion beam may exit from said ionisation chamber along a predetermined ion beam direction,

means for establishing an acceleration field generally along said ion beam direction to establish the exit energy of ions passing through said exit slit,

means defining first and second spaced entrance slits for receiving ions of predetermined masses incident thereon from paths of predetermined different first and second radii respectively, means for establishing a deflecting magnetic field embracing said paths for establishing a circular path for ions from said exit slit toward said entrance slits of radius related to the magnitude of said acceleration field, means for establishing a correcting field in a direction transverse to said acceleration field for guiding ions within a selected mass range related to the magnitude of said acceleration field through said exit slit, and

means for selectively adjusting said acceleration field magnitude to a first magnitude simultaneously with adjusting said correcting field to a first value to produce ions moving along said path of said first radius through said first entrance slit and for adjusting said acceleration field magnitude to a second magnitude simultaneously with adjusting said correcting field to a second value to produce ions moving along said path of said second radius through said second entrance slit.

2. A mass spectroscope in accordance with claim 3 and further comprising means for establishing a focusing field for focusing ions passing through said first and second entrance slits upon first and second ion collecting electrodes respectively, said first and second ion collecting electrodes, and means for selectively adjusting said focusing field to a first focusing value simultaneously with adjusting said acceleration field to said first magnitude and for adjusting said focusing field to a second focusing value simultaneously with adjusting said acceleration field to said second magnitude whereby the energized one of said first and second ion collector electrodes receives a focused bundle of ions.

References Cited UNITED STATES PATENTS 6/1953 Leland.

2/1956 Berry 250-4193 

